During operation of certain types of electrical applications, the voltages generated may exceed the voltage of the DC power supply in use. Thus, current flows from the electrical application into the DC power supply. This may result in damage to the DC power supply in, for example, electrical applications with inductive loads such as DC motors.
In particular, brushless DC motors (BLDC) require commutation for correct and efficient operation. Commutation is achieved in BLDC motors by controlling several switches configured to switch ON and OFF at predetermined intervals and thus controls the commutation of the stator coils of the BLDC motor. Conventional BLDC systems may employ such switches as a semiconductor switch implemented on an integrated circuit (IC).
In most electrical applications, when the electrical application is turned OFF, the DC power supply is isolated from the electrical application. However, in BLDC motors back an electric and magnetic field (EMF) is generated by the commutation of the stator coils. When the BLDC motor is turned OFF or when there is a loss of power, the back EMF generated can induce current that is driven back into the DC power supply. The DC power supply may thus be damaged by the induced current. Such electrical applications thus require electrical isolation from the DC power supply to prevent damage to the power supply during normal operations. Typically, electrical isolation is accomplished by a Shottky diode or MOSFET between the power supply and the electrical application.
FIG. 1 depicts a conventional implementation of an electrical isolation for BLDC motor 20 using MOSFET 12. Using this implementation, there is a low voltage drop across the MOSFET 12 that keeps the voltage supplied to the electrical application high. The power dissipation by MOSFET 12 is also relatively low. However, this implementation does not fully prevent the back flow of induced current into DC power source 10. When the MOSFET 12 is switched OFF, electrical isolation occurs and induced current may not flow from BLDC motor 20 back into DC power source 10. However, when MOSFET 12 is switched ON during normal operations, any induced current due to back EMF in BLDC motor 20 can still back flow into DC power source 10.
Referring to FIG. 2, another conventional implementation of electrical isolation for BLDC motor 20 is depicted using a Shottky diode 17. Shottky diode 17 allows only uni-directional current flow and thus results in electrical isolation of DC power source 10 from the electrical application, in this case BLDC motor 20. However, using Shottky diode 17 results in a high voltage drop across Shottky diode 17 as well as high power dissipation.
There is therefore a need for an improved system of providing electrical isolation for applications that may generate voltages in excess of the DC power supply.